Wide ranges of medical treatments have been developed using implantable devices such as endoluminal prostheses, which are medical devices adapted for temporary or permanent implantation within a body lumen, including naturally occurring and artificially made lumens. Examples of lumens in which endoluminal prostheses may be implanted include arteries such as those located within coronary, mesentery, peripheral, or cerebral vasculature; veins; gastrointestinal tract; biliary tract; urethra; trachea; hepatic shunts; and fallopian tubes. Various types of endoluminal prostheses have also been developed with particular structure to modify the mechanics of the targeted luminal wall.
Stents are one example of an endoluminal prosthesis. Stents are generally cylindrical shaped devices that are radially expandable to hold open a segment of a blood vessel or other anatomical lumen after implantation into the body lumen. Stents have been developed with coatings to deliver drugs or other therapeutic agents. Various types of stents are in use, including balloon expandable and self-expanding stents. Balloon expandable stents generally are conveyed to the area to be treated on balloon catheters or other expandable devices. For insertion, the stent is positioned in a compressed configuration along a delivery device. The stent may be fixed to a balloon that is folded or otherwise wrapped about a guide catheter that is part of the delivery device. After the stent is positioned across a lesion, the stent is expanded by the delivery device. For a self-expanding stent, a sheath is retracted allowing expansion of the stent.
Stents are used in conjunction with balloon catheters in a variety of medical therapeutic applications including intravascular angioplasty. For example, a balloon catheter is inflated during PTCA (percutaneous transluminal coronary angioplasty) to dilate a stenotic blood vessel. The stenosis may be the result of a lesion such as a plaque or thrombus. After inflation, the pressurized balloon exerts a compressive force on the lesion thereby increasing the inner diameter of the affected vessel. The increased interior vessel diameter facilitates improved blood flow. Unfortunately, significant proportions of treated vessels re-narrow or collapse soon after the procedure.
To prevent acute vessel narrowing or collapse, short flexible cylinders, or stents, constructed of metal or various polymers are implanted within the vessel to maintain lumen diameter. The stents acts as a scaffold to support the lumen in an open position. Balloon-expandable stents are mounted on a collapsed balloon at a diameter smaller than when the stents are deployed. Stents can also be self-expanding, growing to a final diameter when deployed without mechanical assistance from a balloon or like device.
Stent insertion may cause undesirable reactions such as inflammation, infection, thrombosis, and proliferation of cell growth that occludes the passageway. Stents have been used with coatings to deliver drugs or other therapeutic agents to the site of the stent and assist in preventing these conditions. In some drug delivery stents, a drug coating is applied to a stent framework. The coating can be applied as a liquid containing the drug or other therapeutic agent dispersed in a polymer/solvent matrix. The liquid coating then dries to a solid coating upon the stent. The liquid coating can be applied by dipping or spraying the stent while spinning or shaking the stent to achieve a uniform coating. Combinations of the various application techniques can also be used.
One problem that has arisen with coated stents is increasing the volume of therapeutic solutions contained on the stent. Often pores or nanopores in the surface of the stent are used to increase the volume of therapeutic solution held by the stent. However, gas bubbles can be trapped in the pores during the coating process. This is particularly true when the pores are micropores or nanopores in the stent surface. The gas bubbles cling to the small pores, and the surface tension of the coating can trap the gas bubble within the nanopore. The gas bubbles in the pores reduce the pore volume available to hold a therapeutic agent, reducing the amount of therapeutic agent that can be dispensed to the patient. This additionally increases the uncertainty in the amount of therapeutic agent actually loaded in the stent and the uncertainty of the amount of therapeutic agent actually dispensed to the patient.
It would be desirable to have an implantable device coating system and method that would overcome the above disadvantages.